1. Field of the Invention
The present invention relates to an automotive alternator comprising a rectifier for converting an alternating current generated in a stator coil into a direct current.
2. Description of the Related Art
FIG. 10 is a cross-section of a conventional automotive alternator. This alternator includes: a case 3 comprising an aluminum front bracket 1 and an aluminum rear bracket 2; a shaft 6 disposed in the case 3 to one end of which a pulley 4 is secured; a Lundell-type rotor 7 secured to the shaft 6; a stator 8 secured to the inner wall of the case 3; slip rings 9 secured to the other end of the shaft 6 for supplying electric current to the rotor 7; a pair of brushes 10 moving in contact with the slip rings 9; a brush holder 11 accommodating the brushes 10; a rectifier 12 electrically connected to the stator 8 for converting alternating current generated in the stator 8 into direct current; a heat sink 17 fitted over the brush holder 11; and a regulator 18 attached to the heat sink with adhesive for adjusting the alternating current generated in the stator 8.
The rotor 7 includes: a rotor coil 13 for generating magnetic flux by passing electric current therethrough; and a pole core 14 disposed so as to cover the rotor coil 13 in which magnetic poles are produced by the magnetic flux generated by the rotor coil 13. The pole core 14 includes a first pole core assembly 21 and a second pole core assembly 22 which mutually interlock. Centrifugal fans 5 for cooling are welded to the axial ends of the first pole core assembly 21 and second pole core assembly 22.
The stator 8 includes: a stator core 15; and a stator coil 16 composed of wire wound onto the stator core 15 in which an alternating current is generated by changes in the magnetic flux from the rotor coil 13 as the rotor 7 rotates.
The rectifier 12 includes: a positive-side heat sink 24 having a plurality of fins 24a on the reverse side arranged in an arc shape; four positive-side diodes 23 secured by soldering to the upper surface of the positive-side heat sink 24; an arc-shaped negative-side heat sink 26; four negative-side diodes 25 secured by soldering to the negative-side heat sink 26; and a circuit board 27 for electrically connecting each of the diodes 23, 25 to the stator coil 16, the rectifier 12 converting the three-phase alternating current generated by the stator 8 into a direct current.
The positive-side heat sink 24 and the negative-side heat sink 26 are disposed on a generally flat plane intersecting the shaft 6 perpendicularly, and are housed inside the case 3. The positive-side heat sink 24 and the negative-side heat sink 26 are composed of aluminum which has high thermal conductivity, and the radially outer negative-side heat sink 26 is grounded by direct attachment to the case 3.
The positive-side diodes 23 and negative-side diodes 25 have square bases in both cases, and the overall shape thereof is formed by molding resin in a rectangular shape in order to protect connecting portions of lead terminals projecting from one side thereof.
In a vehicle alternator of the above construction, a current is supplied by a battery (not shown) through the brushes 10 and slip rings 9 to the rotor coil 13, whereby magnetic flux is generated, giving rise to a magnetic field and at the same time, the pulley 4 is driven by the engine and the rotor 7 is rotated by the shaft 6, so that a rotating magnetic field is imparted to the stator coil 16 and electromotive force is generated in the stator coil 16. This alternating electromotive force passes through the positive-side diodes 23 and the negative-side diodes 25 of the rectifier 12 and is converted into direct current, the magnitude thereof is adjusted by the regulator 18, and the battery is recharged.
While the alternator is generating power, the rotor coil 13, the stator coil 16, the positive-side diodes 23, the negative-side diodes 25, and the regulator 18 are constantly generating heat. For example, in an alternator with a rated output current in the 100 A class, the amount of heat generated is 60 W in the rotor coil 13, 500 W in the stator coil 16, a total of 120 W in the positive-side diodes 23 and the negative-side diodes 25, and 6 W in the regulator 18. The excessive generation of heat causes deterioration in the performance of the alternator and reduces the working life of the parts.
For that reason, the fans 5 are rotated together with the rotation of the rotor 7, external air is introduced into the case 3 from openings A in the case 3 by this rotation, and the external air flows as indicated by the arrow M in FIG. 10, cooling the negative-side heat sink 26, the negative-side diodes 25, the positive-side heat sink 24, and the positive-side diodes 23. The external air then flows radially outwards from the fans 5, cools the end portions of the stator coil 16 in the rear end, and is expelled to the outside through openings B.
External air is also introduced into the case 3 from openings C by the rotation of the fans 5, and the external air flows as indicated by the arrow N in FIG. 10, cooling the power transistors of the regulator 18. The external air then flows radially outwards from the fans 5, cools the end portions of the stator coil 16 in the rear end, and is expelled to the outside through openings D.
Similarly, external air introduced through openings E in the front bracket 1 flows radially outwards from the fans 5, cooling the end portions of the stator coil 16 in the front end. The external air is then expelled outside the case 3 through openings F.
In an automotive alternator of the above construction, the lead wires of the positive-side diodes 23 and the lead wires of the negative-side diodes 25 are disposed so as to face each other, in other words, the positive-side diodes 23 and the negative-side diodes 25 are radially aligned with each other in order to facilitate electrical connection of the lead wires of the positive-side diodes 23 and the lead wires of the negative-side diodes 25 to the circuit board 27.
In this case, a problem has been that although the circuit board 27 and the lead wires of each of the diodes 23, 25 can be connected without taking up space and the electrical connection is easily made, because the longitudinal axes of the negative-side diodes 25 are lined up radially, the radial dimension of the radially outer negative-side heat sink 26 is enlarged, and the radial dimension (Y in FIG. 12) of the heat sink 26 is the same as or greater than the radial dimension (X in FIG. 12) of the radially inner positive-side heat sink 24 (Y&gt;X), making size reductions impossible.
Furthermore, another problem has been that after the external air passes through the air gaps between the negative-side diodes 25 as indicated by the Arrows P in FIG. 12, it passes through the air gaps between the positive-side diodes 23, but since the volume of air which can pass through is determined by the dimensions of the air gaps between the positive-side diodes 23, unwanted spaces exist within the radially outer negative-side heat sink 26 which do not contribute to increasing the passage of external air, that is, which do not contribute to improving the cooling of the negative-side diodes 25.
Yet another problem has been that the outer circumferential side of the negative-side heat sink 26 is the side from which the external air is introduced and the temperature of the external air is low there, the temperature of the negative-side heat sink 26 rising towards the inner circumferential side, but because the longitudinal axes of the negative-side diodes 25 are lined up radially, the radial dimension of the radially outer negative-side heat sink 26 is enlarged, and the high-temperature regions of the negative-side heat sink 26 occupy a correspondingly larger portion, causing certain points on the inner circumferential side of the negative-side diodes 25 to have locally high temperatures.